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Water, carbon, and climate on the roof of the world

Namtso Lake, which is the largest lake in the Tibet Autonomous Region, lies about 70 miles (110 kilometers) northwest of Lhasa at an elevation of 15,479 feet (4,718 meters). (Photo by Jia Hu.)

March 1, 2011 | A study led by NCAR postdoctoral researcher Jia Hu and Julia Klein from Colorado State University looks at the complicated relationship between plants, water, carbon, and climate on the Tibetan Plateau.

With an average elevation of more than 14,700 feet (4,500 meters), the Tibetan Plateau is the world’s largest and highest. It is also warming at a rate twice that of the global average, with accompanying changes in precipitation patterns. Historically, about 80% of the plateau’s precipitation has come from monsoon rains and 20% from snow. In recent years, the frequency of heavy winter and spring snowstorms has increased; such storms can cover vegetation for extended periods, leading to livestock starvation. Along with rising air temperatures, increases in snow may have profound effects on the plateau’s vegetation and net exchange of carbon dioxide with the atmosphere.

During a 2009–2010 field experiment at Namtso Research Station, located in an alpine meadow on the plateau, the researchers focused on how increased snowfall and higher air temperatures affect the ways that plants use water. By placing fiberglass warming chambers on the ground, they raised air temperatures about 2°C (3.6°F). They also applied the equivalent of one meter (39 inches) of fresh snow during April of each year to mimic a spring snowstorm. They collected root samples throughout the growing season from plant groups with different rooting depths, along with soil and groundwater samples. They also measured daytime carbon dioxide fluxes.

Hu and colleagues found that precipitation appears to be more important than temperature in shaping how the Tibetan Plateau will exchange carbon with the atmosphere in a future climate. The artificial increase in air temperatures did have a negative effect on carbon dioxide uptake, but this is most likely due to decreases in plant productivity under drier soil conditions; the additional snow increased carbon dioxide uptake by enhancing plant productivity. The highest carbon uptake rates occurred in late July, suggesting that both warmer air temperatures and high soil moisture levels (due to the monsoon) contribute to higher rates during this time.

“The study suggests that while snowstorms can be detrimental to the livelihoods of the herders living on the plateau, the increase in snow may actually benefit the ecosystem during the summer by providing extra water for shallow-rooted plants,” says Hu. “The increase in snow also appears to mitigate the effects of warming alone on carbon dioxide uptake.”